1. Technical Field
The invention is directed to a process for device fabrication in which an energy sensitive resist material is used.
2. Art Background
Devices such as integrated circuits are complex structures made of a variety of materials. These materials are precisely configured to form the desired device by a variety of processes. A lithographic process is frequently used to transfer the desired configuration into a substrate to fabricate such devices.
Lithographic processes use intermediate materials frequently referred to as resists. A positive or negative image of the desired configuration is first introduced into the resist by exposing it to patterned radiation which induces a chemical change in the exposed portions of the resist. This chemical change is then exploited to develop a pattern in the resist, which is then transferred into the substrate underlying the resist.
The efficacy of a lithographic process depends at least in part on the resist used to transfer the pattern into the substrate. Certain types of resists offer particular advantages in the context of specific lithographic processes. For example, solution-developed resists are designed to have absorption characteristics appropriate for use at certain exposure wavelengths. It is axiomatic that, if the resist material is opaque to the exposing radiation, the exposing radiation will not be transmitted into the resist material and the desired chemical change will not occur. Therefore it is important to select a resist material that has the appropriate light transmission characteristics at the wavelength of the exposing radiation. Other considerations that drive the selection of an appropriate resist material include the etch resistance of the resist after it has been exposed and developed.
A variety of resist materials are employed in lithographic processes for device fabrication. One class of resist materials contains a polymer which has certain functional groups (e.g. alcohol (OH); phenol (C.sub.2 H.sub.5 OH); carboxylic acid (COOH); etc.). A certain portion of these functional groups are "masked," i.e., the hydrogen atom is removed and replaced by moieties referred to as protecting groups. These protecting groups are removable from the polymer by acidolysis or hydrolysis. A polymer with a significant number of these protecting groups has a very different solubility in developer solutions (typically aqueous base solutions) than a polymer with substantially fewer of these protecting groups. Examples of protecting groups include acetals, ketals, bis(trimethylsilylmethyl)methyloxy, t-butoxycarbonyloxy, t-butyl esters, and t-butyl ethers which are cleavable from the functional groups by acidolysis or hydrolysis. The functional groups from which the protecting groups have been cleaved are referred to as unmasked functional groups.
The resist materials also contain an energy-sensitive material in combination with the polymer. When exposed to a certain energy (energy of a particular wavelength (e.g. 248 nm) or type (electron beam)) a moiety is generated from the energy-sensitive material which effects the cleavage of the protecting group, thereby "unmasking" the functional group. When the protecting group is an acid labile group, i.e. it is removed in the presence of acid, the energy sensitive material is typically a photoacid generator (PAG). The greater the number of protecting groups that are cleaved from the polymer, the greater the chemical contrast between the polymer exposed to radiation and the polymer not exposed to radiation. This chemical contrast between the unexposed resist material and the exposed resist material is exploited to develop a pattern in the resist material.
The chemical contrast is typically a difference between the solubility of the exposed resist compared to that of the unexposed resist in a developer solution. The developer solution is typically an aqueous base solution. Ideally, all of the resist material in one area (the exposed area in a positive resist, the unexposed area in a negative resist) is removed and the resist material in the other area remains. In positive resists in which the basis for contrast is the relative solubility of the exposed and unexposed resist in developer solution, however, there is a tendency for the moiety that effects deprotection to diffuse from the exposed resist into the unexposed resist. Such diffusion, if significant enough, will erode the developed features. The result is features that do not have the desired dimensions. This problem is exacerbated the longer the time that elapses between exposure and development, because, over time more of the moiety that effects deprotection diffuses into the unexposed region.
In integrated circuit devices, feature sizes are decreasing (0.35 .mu.m to 0.25 .mu.m to 0.18 .mu.m, etc.) in order to increase the number of semiconductor devices that can be formed on an integrated circuit chip. As feature size decreases, the amount of feature erosion that is acceptable also decreases. Therefore solution-developed resist materials are sought that reduce the degree to which features are eroded during imaging and development.